Identifying Precipitation Types and Intensity Changes

Overview

The Federal Aviation Administration (FAA) Weather Observation Improvements (WOI) program manages the evolution of the existing aviation weather observation sensor network to one that provides the optimal quantity and quality of ground, air, and space-based sensors. Of primary focus is the surface weather sensor network in the airport terminal environment. Accurate knowledge of airport weather conditions is critically important for aircraft and airline operations (e.g., determining aircraft deicing holdover times as well as takeoff and landing performance calculations) and airport traffic management (e.g., selection of appropriate runways and estimating airport arrival and departure capacity). The nation’s current ground-based airport weather observation architecture has evolved over decades in different ways at different airports, leading to inconsistencies and shortfalls in airport weather sensing and data dissemination. The purpose of the information provided on this page is to provide visual examples of the different types of winter precipitation that can be encountered at airports around the country.

The Automated Surface Observing System (ASOS)/Automated Weather Observing System (AWOS) primarily serve as the source on-airport sensor and algorithmically determine a precipitation type and intensity. The precipitation type sensor on the ASOS has reached its end of life and the latest generation of present weather sensors must be tested to determine their capabilities for correctly identifying and reporting the types of precipitation that are occurring. To do this, high resolution video cameras have been collocated with the new sensors to record the various weather conditions for later comparison to the sensor data. Examples of the different types of precipitation recorded by the cameras are shown below, which would be used for comparison against the sensor data. 

[Full Playlist]

Snow Graupel Ice Pellets Rain Drizzle Mixed

Snow

IMPACTS

Any observations of snowfall require an aircraft undergo deicing operations. Light and moderate intensity snow (-SN, SN) have holdover times (HOTs) that tell pilots and ground deicing crews how long their anti-icing fluids will last under the given conditions. Heavy snow (+SN), which occurs in conditions where the visibility drops below 5/8 of a mile or the liquid water equivalent of the falling snow is greater than 0.1 inches per hour, has no holdover times and often results in an airport shutting down in more extreme conditions. Snowfall of any amount can also have impacts on runway plowing operations, which can reduce the number of takeoffs and landings due to runway closures while plowing and and chemical application operations are occurring.

SNOW BEGINS

  • Dry Target
    • Look for multiple opaque flakes/pellets on the target (Day/Night)
    • Speed slide video to determine good start time (D/N)
  • Wet Target
    • Look for opaque flakes/pellets on target (D/N)
    • Speed slide video to determine good start time (D/N)
    • Wide views reduction in visibility (D/N)
  • Note:  Floating motion (D/N)
  • Pellets that are opaque slightly bounce and roll (D/N)
  • Note: Shows up much brighter than rain (D/N)

SNOW INCREASING

  • More opaque pellets/flakes accumulate on target/ground (D/N)
  • Wide view visibility drop (D/N)
  • Pause video look for increase in number of opaque pellets/flakes per volume (D/N)
  • Speed slide video to determine increase time (D/N) (Static Cameras)
  • Increase of snowflake size (D/N)

SNOW DECREASING

  • Slowing of opaque pellets/flakes accumulating on target/ground (D/N)
  • Wide view visibility increase (D/N)
  • Pause video look for decrease in number of opaque pellets/flakes per volume (D/N)
  • Speed slide video to determine decrease time (D/N) (Static Cameras)
  • Decrease of snowflake size (D/N)

SNOW ENDS

  • No more fall streaks (D/N)
  • No more accumulation (D/N)
  • Speed slide video to determine good end time (D/N)

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Snow Graupel Ice Pellets Rain Drizzle Mixed

GRAUPEL

GRAUPEL BEGINS

GRAUPEL ENDS

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Snow Graupel Ice Pellets Rain Drizzle Mixed

Ice Pellets

ICE PELLETS BEGIN

  • Dry Target
    • Look for translucent pellets bouncing on the target (Day/Night)
    • Speed slide video to determine good start time (D/N)
  • Wet Target
    • Look for translucent pellets sticking on target (D/N)
    • Look for drops and floating ice in puddles (D)
    • Look for drops in reservoir, if reservoir is filled (D/N)
    • Speed slide video to determine good start time (D/N)
  • Look at pellets bouncing off of objects (Roaming Camera) (D/N)
  • Note: Zoom into videos to see bouncing clearer (D/N)

ICE PELLETS INCREASING

  • Wide view visibility drop (D/N)
  • Pause video look for increase in number of translucent pellets per volume (D/N)
  • Speed slide video to determine increase time (D/N) (Static Cameras)
  • Increase of bouncing off objects/target (D/N)
  • Look for increase of translucent pellets and splashing, if reservoir is filled (D/N)

ICE PELLETS DECREASING

  • Wide view visibility increase (D/N)
  • Pause video look for decrease in number of translucent pellets per volume (D/N)
  • Speed slide video to determine decrease time (D/N) (Static Cameras)
  • Decrease of bouncing off objects/target (D/N)
  • Look for decrease of translucent pellets and splashing, if reservoir is filled (D/N)
  • Note: Zoom into videos to see bouncing clearer (D/N)
  • Note: Different cores Liquid splat, solid bounce (D/N)

ICE PELLETS END

  • No more fall streaks (D/N)
  • No more accumulation (D/N)
  • No more splashing (D/N)
  • Speed slide video to determine good end time (D/N)

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Snow Graupel Ice Pellets Rain Drizzle Mixed

Rain

RAIN BEGINS

  • Dry Target
    • Look for multiple drops on the target (Day/Night)
    • Speed slide video to determine good start time (D/N)
  • Wet Target
    • Look for drops on target (D/N)
    • Look for drops in puddles (D)
    • Look for drops in reservoir, if reservoir is filled (D/N)
    • Speed slide video to determine good start time (D/N)
  • Note: Fall streak tends to be straight (D/N)

RAIN INCREASING

  • Larger drops hitting target (D/N) 
  • More splashing in puddles (D)
  • More splashing in reservoir if filled (D/N)
  • Pause video look for increase in number of drops per volume (D/N)
  • Speed slide video to determine increase time (D/N) (Static Cameras)
  • Target saturation increases (D/N)

RAIN DECREASING

  • Larger drops diminish (D/N) 
  • Less splashing in puddles (D)
  • Less splashing in reservoir if filled (D/N)
  • Pause video look for decrease in number of drops per volume (D/N)
  • Speed slide video to determine decrease time (D/N) (Static Cameras)
  • Target saturation decreases (D/N)

RAIN ENDS

    • No more fall streaks (D/N)
    • No more splashing puddles (D)
    • No more splashing reservoir (D/N)
    • Speed slide video to determine good end time (D/N)

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Snow Graupel Ice Pellets Rain Drizzle Mixed

Drizzle

DRIZZLE BEGINS

  • Dry Target
    • Look for small/uniform particles accumulation on target (Day/Night)
    • Speed slide video to determine good start time (D/N)
  • Wet Target
    • Look for small/uniform particles accumulation on target (D/N)
    • Look for no drops in reservoir, if reservoir is filled (D/N)
    • Speed slide video to determine good start time (D/N)
  • Look for reduced visibility  (D/N)
  • Note: Floating motion (D/N)

DRIZZLE INCREASING

  • Look for more small/uniform particles on target (D/N)
  • Look for more coalescence on target producing more frequent fall streaks (D/N)
  • Look for reduction of visibility (D/N)
  • Speed slide video to determine increase time (D/N) (Static Cameras)
  • Volume in camera view increase (pause) (D/N)

DRIZZLE DECREASING

  • Look for less small/uniform particles on target (D/N)
  • Look for less coalescence on target producing less frequent fall streaks (D/N)
  • Look for increase of visibility (D/N)
  • Speed slide video to determine decrease time (D/N) (Static Cameras)
  • Volume in camera view decrease (pause) (D/N)

DRIZZLE ENDS

  • No more fall streaks (D/N)
  • No more accumulation (D/N)
  • Speed slide video to determine good end time (D/N)

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Snow Graupel Ice Pellets Rain Drizzle Mixed

MIXED

RAIN/SNOW COMBINATION

  • Noticeable drops hitting target
  • Splashing in puddles/target reservoir
  • Opaque snowflakes/pellets hitting and possibly sliding down target
  • Snowflakes may show up considerably larger than falling rain drops

ICE PELLET/RAIN/SNOW COMBINATION

  • Noticeable drops hitting target
  • Splashing in puddles/target reservoir
  • Opaque snowflakes/pellets hitting and sticking to target
  • Snowflakes may show up considerably larger than falling rain drops
  • Look for translucent pellets bouncing off the target/objects/ground

SNOW/ICE PELLET COMBINATION

  • Opaque snowflakes/pellets hitting and sticking to target
  • Look for translucent pellets bouncing off the target/objects/ground

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